Recent work by Birge and Low has deomonstrated that, in Escherichia coli, two of the three best studied recombination deficiency mutuations which are known to cause increased sensitivity to ionizing radiation are also associated with the appearance of partially recombinant DNA molecules. The proposed project is an attempt to remove these recombinational intermediate structures from the cell and to characterize their structure by electron microscopy. In addition, a further search for new recombination deficiency mutations which might produce other types of intermediate forms of recombinant DNA is proposed. It is hoped that these studies may eventually lead to a sequencing of the steps involved in the production of recombinant DNA molecules and elucidate one potential mechanism for DNA repair.